Control Effectiveness Of Nonspecific Predators On Pomacea Canaliculata And Their Interaction Mechanisms

Golden apple snail Pomacea canaliculata,is a cosmopolitan harmful alien invasive species listed by the World Conservation Union.It was introduced to China more than 30 years and has been widely distributed in southern China,and caused incalculable economic losses and ecological harms on rice production and aquatic biodiversity.Molluscicides are the first choice for most farmers but result in severe environmental pollution and food insecurity.Searching for environment-friendly technologies on controlling the snails was urgent,and biological control is likely the proverbial preferable strategy because of its sustainability and potential ecological benefits.However,except duck has been widely used to control P.canaliculata in paddy fields,biocontrol resources of P.canaliculata with effective control on snail harm and the related research are limited.For this purpose,different types of biocontrol resources were screened out,including ornamental animal,economic animal and biocontrol plant.The predatory and application potentials of these biocontrol species were investigated.Moreover,from the perspective of predator-prey or herbivore-plant interactions,the control mechanisms of these biocontrol resources on P.canaliculata and the response mechanisms and strategies of P.canaliculata against predators were assessed.The main results are as follows:?1?Six biological species in three types on controlling P.canaliculata were screened out,including ornamental animal?such as spotted green pufferfish Tetraodon nigroviridis and assassin snail Clea helena?,economic animal?such as leech Whitmania pigra,loach Misgurnus anguillicaudatus and red-eared turtle Trachemys scripta elegans?and biocontrol plant?duckweed Lemna minor?.The pufferfish exhibited an ideal predatory potential on P.canaliculata under brackish water condition.The survival rate of 50 snails?including 10adults?was less than 25%in the presence of pufferfish for 7 days,and this predatory effect was more profound in brackish water than in fresh water.The snail C.helena and P.canaliculata were kept in the same aquatic habitat for one week,the mortality of P.canaliculata increased to 53.3%.The leech W.pigra is suitable to control P.canaliculata in rice fields.More than 80%of the 80 snails?of which half had reached maturity?were preyed upon by 20 leeches?2-7 g?in miniature rice fields over a 15-day period,and the number of rice seedlings damaged by P.canaliculata decreased more than 50%.The predatory potential of loach M.anguillicaudatus against neonate snail P.canaliculata was not ideal,and approximately 2 snails per day were preyed upon by a loach.The turtle T.scripta elegans is capable of preying on P.canaliculata with various sizes,but escaping-prevention facilities should be taken in practice.The duckweed L.minor can mitigate harms of P.canaliculata on rice after introduced to rice fields.More than 60%of snails preferred to feed on L.minor,when the rice plants were 15,23 or 30 days old.In a simulated paddy habitat,the amount of rice eaten by the snails decreased more than 10%after the duckweed was introduced for 3 days.?2?The control mechanisms of biocontrol resources on P.canaliculata were summarized.The results showed that the survival,growth and behavior of P.canaliculata were affected by direct,indirect and diversified effects of biocontrol resources in the following respects.First,direct lethal mechanism that predators kill the snail by bite the whole body like T.nigroviridis or suck the soft tissues like W.pigra or otherwise.Second,direct damage mechanism that predators increase the possibility of snail death by breaking the shell or gnawing on the soft parts,such as T.nigroviridis and T.scripta elegans.Third,indirect lethal mechanism that a long-term predation risk leads to the snail death,for example,only 26.7%of juvenile snails?7-12 mm?surviving under predation risk was produced by T.scripta elegans for 25 days.Fourth,direct behavioral disturbance mechanism that predators affect the snail behavior by bite or disturbance,for instance,the adult snails barely feed,crawl and copulate in the presence of T.nigroviridis.Fifth,indirect behavioral disturbance mechanism that the snail behavior influenced by predation risk,for example,the food intake of P.canaliculata was decreased and growth was restricted under predation risk produced by T.scripta elegans and W.pigra.Sixth,replacement control mechanism that the decoy effect of biocontrol plants such as duckweed L.minor on P.canaliculata with their good palatability to reduce the snails harm on target rice crops.?3?For further understanding the control effectiveness of predators on P.canaliculata and the mechanisms for successful invasion of P.canaliculata,the possible response mechanisms and strategies of P.canaliculata against predators were explored from the aspects of survival,growth,reproduction,behavior,morphology and physiology.The results indicated that P.canaliculata exhibited multiple response mechanisms and strategies in the presence of predators as follows.First,escape strategy that P.canaliculata can crawl out the water or self-bury in sediment to avoid predators,for example,more than 25%and40%of P.canaliculata crawled out the water under the predation of M.anguillicaudatus and C.helena,respectively.Second,in-situ defensive strategy that more than 95%of snails closed their opercula tightly in the presence of T.nigroviridis.Third,growth-reproduction trade-off mechanism that P.canaliculata female adults produced eggs earlier and increased reproductive investment in response to predation risk from caged T.scripta elegans at the expense of slowed growth.Fourth,behavioral-morphological defense trade-off mechanism that P.canaliculata male adults showed stronger anti-predator defense capability than females in shell morphological features,but the females exhibited faster escape reaction to avoid predators.Fifth,reproduction-defense trade-off mechanism that female adults require substantial CaCO₃ to produce egg shells and embryonic shell growth before hatching at the cost of their own shell defense,and the shell strength of males increased more than females in response to predation risk.Sixth,physiological response mechanism that P.canaliculata juveniles and male adults activated their antioxidant systems with peroxidation damage under predation risk,but the antioxidant system of female adults was not activated.In conclusion,this study focused on six biocontrol species that were screened out to control P.canaliculata,explored the interaction mechanisms of P.canaliculata and these biocontrol resource species.The control effectiveness and mechanisms of biocontrol resource species on P.canaliculata were ascertained,and the mechansims and strategies of P.canaliculata in response to predators were revealed.The findings would provide theoretical foundation and practical references for further applying these biocontrol techniques to agriculture production and for snail-controlling potential and its feasibility assessment.